Dental implant system

ABSTRACT

A dental implant assembly is provided, as well as a system and method for exposing an embedded implant after osseointegration has taken place. The implant assembly comprises an implant member for embedding in the jaw and a rest factor member for securing to the implant member, the rest factor member having an upper rest surface just above the tissue level for opposing an overlying portion of a prosthesis anchored elsewhere in the jaw to form a non-retentive rest or support for accepting down pressure from the prosthesis. The implant member is relatively short and can be installed in distal jaw regions without interference with the mandibular nerve. A bore is cut out in the jaw for receiving the implant, inserting the implant and an attached healing screw in the implant. The implant site is closed and osseointegration takes place over an extended period. Subsequently, the implant site is uncovered, the healing screw is removed, and the rest factor member is secured in the implant.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a Continuation-In-Part of application Ser. No.07/861,183 filed Mar. 31, 1992, abandoned, which was a Continuation ofapplication Ser. No. 07/751,661 filed Aug. 22, 1991, now U.S. Pat. No.5,254,005, which was a Continuation of application Ser. No. 07/436,432filed Nov. 14, 1989 (now abandoned).

BACKGROUND OF THE INVENTION

The present invention relates to a dental implant system.

Dental implants are embedded in the jaw bone and serve to anchor one ormore artificial teeth or dentures. Most implant systems involve arelatively long implant cylinder which is placed into a custom boredhole in the jawbone, then left for several months to allow healing andbone integration. Then the implant must be exposed for attachment of adental prosthetic appliance such as a crown, denture, partial denture orbridge. This generally involves the dentist cutting out a flap of tissuewhich is peeled back to expose the implant, and secured by sutures afterinstalling the prosthesis. This results in a relatively large area oftrauma with a certain degree of pain to the patient and risk ofpost-operative infection.

Another problem with conventional implants is their length, which makesthem difficult to implant in the distal jaw region, where there isinsufficient depth to enable their insertion without interference withthe mandibular nerve, without the assistance of a dental surgeon tolocate the precise position of the nerve and ensure that the implantdoes not interfere with it. A shorter cylindrical implant would notnormally be suitable since it would provide insufficient “hold” andwould likely become loosened with time if anchored to a denture orbridge. Also, side to side forces on the implant lead to bone erosionand trauma. Thus, dentures or bridges are often not anchored at the rearof the jaw. However, this has the disadvantage that trauma to the tissueand underlying bone beneath the denture occurs as a result of thedenture repeatedly impacting the bone, particularly with long dentureswhich will tend to tilt or rotate about their attachment or anchorpoints during chewing or other jaw motions. This biting pressure canresult in bone erosion or resorption down to the level of the nerve.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved dental implantsystem and method which is less likely to cause significant tissuetrauma and which reduces bone erosion as a result of denture wear.

It is a further object of this invention to provide an improved systemand method for exposing an implant site after osseointegration has takenplace.

According to a first aspect of the present invention, a dental implantassembly is provided which comprises a first, implant member forimplanting in the distal region of the jaw bone, and a second member orrest factor for attachment to the implant member. The two members havecooperating releasable securing devices for releasably securing themtogether, preferably comprising a screw threaded bore in the implantmember and a corresponding threaded portion on the rest factor member.The rest factor member projects up to just above the level of the tissueoverlying the jaw bone and has an upper surface opposing an overlyingportion of a prosthesis anchored elsewhere in the jaw to form a rest forthe prosthesis which accepts down pressure only, and which acts as asupport to prevent or restrict bone erosion. The implant member has aselected height less than the depth of the mandibular nerve at theimplant site, so that it can be embedded in the bone without risk ofinterference with the nerve. At the same time, the implant member is aswide as possible, and preferably has the maximum diameter possibleaccording to the width of the patient's alveolar ridge at the implantsite. The implant member diameter is preferably selected to be 1 mm lessthan the available alveolar ridge or bone width at the implant site.Implant members in a range of different heights and diameters arepreferably provided to meet the requirements of a range of patients.Preferably, implant members with heights of 2 mm, 4.5 mm, 7 mm and 10 mmare provided, to allow for patients whose jawbone is already eroded tosome extent. Implant members with diameters ranging from about 4 mm to 6mm may be provided.

Since the implant member is of relatively large diameter, it has arelatively large surface area resisting downward forces. In a preferredembodiment of the invention, the implant member has a generallycylindrical body with an upper end portion and downwardly depending stemportion which engages in a corresponding recess drilled out in the bone.Preferably, at least part of the stem portion at the lower end of theimplant member has an annular recess forming an outer rim and centralboss. This engages a corresponding annular recess drilled out in thebone to resist sideways movement of the implant. This will resistsideways movement of the implant during osseointegration, and alsoprovides additional depth for securing the rest factor to the implantmember. A bore of corresponding shape to the undersurface of the implantmember is drilled out in the jawbone at the implant site, so that whenthe implant member is positioned in the bore, the peripheral rim willprovide stabilization of the member against lateral movement during theosseointegration period. The shape of the undersurface of the implantprovides a large area of bone to implant contact for osseointegration,and significant resistance to both lateral and downward forces bothduring and after the osseointegration period. Preferably, at least twoseparate or double lead threads are provided, and triple or quadruplethreads may be provided for added retention. Bone grows into the gapsbetween threads.

The outer surface of the stem portion of the implant preferably hasthreads to provide additional surface area for bone attachment. Bonegrows into the area above and below the threads to resist loosening ofthe implant.

Since the rest factor is not anchored to the prosthesis, the risk of jawbone erosion or damage as a result of upward forces is reduced. However,the rest factor does accept down pressure as a result of biting pressureof the denture, and will thus reduce the risk of trauma to the tissueand jawbone erosion as a result of pressure. The localized contactbetween the rest factor and the underlying bone via the implant memberreduces or substantially eliminates pressure trauma on the entire bone.

If desired, the upper surface of the rest factor and the opposingportion of the prosthesis may be provided with opposing, non-retentivemating formations, such as opposing slightly convex and concaveformations, for guiding the prosthesis against the rest factor. However,these formations do not provide any upwards retention of the prosthesis.A series of such rest factors may be provided at appropriate locationsin the jaw where maximum down pressure from a denture is encountered,considerably reducing the discomfort of denture use and reducing therisk of tissue and jawbone damage as would result from conventionallyanchored dentures.

The rest factor may be preformed with a suitable rest surface inincremental heights, in which case the procedure after removing thehealing screw comprises selecting an appropriate height rest factor andsecuring the selected rest factor in the implant member.

The implant member may be relatively short with a relatively largediameter, so that it can be anchored securely in the jawbone withoutneeding a deep bore to be drilled out. The implant member is provided inseveral heights. The shortest of the implants will be shorter and widerthan conventional cylindrical implants, and thus can be used at the backor posterior mandible of the jaw where the nerve position prevents orrestricts the use of long implants. This implant is particularlysuitable for positioning a rest factor in the second molar area inconjunction with implant dentistry where cantilevered bridges oranterior implants need support or in other places where a rest factor isneeded in dentistry. The implant requires less bone to be drilled outthan conventional cylindrical implants, reducing or minimizing boneloss, and is able to accept hundreds of pounds of down pressure from anoverlying denture or prosthesis.

After bone integration, the implant can be recovered by piercing thetissue overlying a healing screw secured to the implant with a pointedend of a locating guide tool, probing the implant site with the pointedend until it engages a hole in the top of the healing screw, insertingthe pointed end into the hole, utilizing a tissue punch centered on theguide tool to cut out a plug of tissue directly over the implant, andsubsequently removing the healing screw.

Thus, the locating tool and tissue punch can be removed together fromthe implant site, carrying with them the tissue plug to expose thehealing screw for removal with a separate tool. The implant is thenexposed for secondary healing or restoration procedures while a minimumamount of tissue has been disturbed and little or no suturing isrequired. This considerably reduces the trauma, secondary tissuehealing, discomfort to the patient, and risk of infection.

The implant member provides osseointegration with good resistance toloosening forces as a result of chewing. It may alternatively be used tosecure other dental devices such as an implant denture anchor or animplant magnet abutment.

An implant locating and exposing tool may be used to recover theimplant. The tool comprises an elongate shaft having a head at one endand a locating probe at the opposite end having a sharp end for probingthe tissue over an implant site to locate a central hole in the top of ahealing screw, and a cutter member mounted on the elongate member withits cutting face facing in the same direction as the locating probe, thecutter member being movable along the elongate shaft to cut out a plugof tissue overlying a healing screw.

Thus, the healing screw can be located and the overlying tissue removedin one step, without having to cut out a relatively large flap oftissue.

The healing screw may be provided with a concave upper surface so thatthe locating probe will be guided towards the central opening.

The implant system and method described above provides a rest surfacefor an overlying cantilevered bridge or denture which is anchoredelsewhere in the jaw, on which the denture can rest and which acceptsdown pressure from the denture, reducing tissue trauma. Rest surfacesmay be provided wherever needed, in conjunction with the conventionalimplants and anchors used for securing the denture or prosthesis in thejaw. The improved implant recovery tool and method produces minimaltrauma when exposing a previously embedded implant for subsequentconnection to either a rest factor or to a conventional anchor or magnetabutment.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the followingdetailed description of a preferred embodiment of the invention, takenin conjunction with the accompanying drawings, in which like referencenumerals refer to like parts, and in which:

FIG. 1 is a perspective view of the separated implant and healing screwcomponents of an implant assembly according to a preferred embodiment ofthe invention;

FIG. 2 is a perspective view of a rest factor of the implant assembly;

FIG. 3 is a sectional view of the implant taken on line 3—3 of FIG. 1;

FIG. 4A is a perspective view of a preferred embodiment of the implantlocating tool according to another aspect of the invention;

FIG. 4B is a perspective view of a healing screw removal tool;

FIG. 5 is a sectional view through a typical jawbone with a finishedimplant in place and a rest factor inserted;

FIG. 6 is a jawbone section illustrating the initial drilling set up foran implant;

FIG. 7 is a similar view illustrating the final counter-boring operationfor an implant;

FIG. 8 is a similar view with an implant and healing screw in place andenclosed under tissue for the osseointegration period;

FIG. 9 illustrates the locating of the integrated implant under theissue;

FIG. 10 illustrates the cutting out of a tissue plug overlying thehealing screw;

FIG. 11 illustrates the removal of the healing screw;

FIG. 12 illustrates the lower half of a patient's jaw with a fulldenture anchored in place and seated on rest factors in posterior areason both sides;

FIG. 13 is a side view of the denture arrangement of FIG. 12;

FIG. 14 is a perspective view of the separated implant and healing screwcomponents of an implant assembly according to a second embodiment ofthe invention;

FIG. 15 is a jawbone section illustrating the initial drilling set upfor the implant of FIG. 14;

FIG. 16 is a similar view to FIG. 15 illustrating the next step in thedrilling procedure;

FIG. 17 is a similar view illustrating the finishing step in the boringoperation;

FIG. 18 is a similar view illustrating the implant and healing screw inplace and enclosed under tissue for the osseointegration period;

FIG. 19 is a perspective view of a rest factor according to anotherembodiment of the invention;

FIG. 20 is a perspective view, partially cut away, of one example of animplant for the rest factor of FIG. 19;

FIG. 21 is a side elevation view, partially cut away, of another implantwith triple retaining threads;

FIG. 22 is a side elevation view of a further implant with quadruplethreads;

FIG. 23 is a top plan view of FIG. 20;

FIG. 24 is a view similar to FIG. 7, showing the addition of a shallowcounter-bore;

FIG. 25 is a similar view showing the cutting of threads to receive animplant;

FIG. 26 is a similar view showing insertion of the implant;

FIG. 27 is a similar view showing attachment of the rest factor;

FIG. 28 is a side elevation view, partially cut away, of anothermodified implant having an alternative wrench element; and

FIG. 29 is a top plan view of the implant of FIG. 28.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1-3 and 5 of the drawings illustrate an implant assembly 10according to a preferred embodiment of the present invention, as well asa preferred method of installing the assembly in the jaw. As bestillustrated in FIG. 5, the implant assembly 10 basically comprises animplant 12 for insertion into a suitably prepared bore 14 in the jawbone16, where it is secured in place by osseointegration as is known in thefield of implant dentistry, and an extension member or rest factor 18secured to the implant 12 and extending up to slightly above the level20 of the tissue or gum 21 to form a rest surface 22 for an overlyingdenture or bridge 24 which is anchored elsewhere in the jaw.

As best illustrated in FIGS. 1 and 3, implant 12 comprises a thin,annular member 26 having a central spigot or downward extension 28projecting from one of its faces and an annular peripheral rim 30projecting from the same face. A central bore 32 extends from theopposite face into the spigot 28, as illustrated in FIG. 4. The bore 32has a larger diameter upper portion 33 having internal screw threads 34extending along its length with a taper 37 at its upper end for addedstrength to hold the threaded engagement. The lower end of bore 32comprises a smaller diameter lower portion 35 extending into spigot 28for centering parts mating with implant 12 and preventingcross-threading of the threaded engagement. The implant will be of metalor any suitably rigid material as is normally used for dental implants,for example surgical titanium alloy. The implant may have spaced, shortgrooves or indents 29 on its outer surface for resisting rotation. Theseare preferably of the order of 1 mm in length.

Also illustrated in FIG. 1 is a healing screw 70 for insertion in theimplant member during the osseointegration process. Healing screw 70 hasa relatively short head portion 72 and downwardly depending shaftportion 73 for engagement in the bore 32 of implant 12. Portion 73 has ascrew threaded larger diameter upper part 74 for threaded engagement inbore portion 33, the upper part 74 having a taper 71 at its upper endmatching the taper 37 at the upper end of the implant bore 32 forsupport and seating of the screw 70 in bore 32. The lower end of portion73 comprises a smaller diameter, cylindrical lower part 75 for fittinginto lower portion 35. The upper end face of the head portion has acentral, tool receiving bore 76 for receiving the end of a suitable toolfor inserting the arrangement in the bore in the jawbone, and forsubsequently receiving the end of a locating or removing tool as will beexplained in more detail below. If desired, the upper end face of screw70 may have a concave or dish-shaped depression 79 surrounding bore 76.The bore corresponding to bore 76 is illustrated in FIG. 11 and has ascrew threaded upper portion 77 and a downwardly depending cylindricalcentering extension 76 corresponding to extension 75 in FIG. 1. Thethreads in upper portion 77 are of opposite hand to those of screwthreaded portion 74. In the embodiment illustrated in the drawings,portion 74 has a right hand thread while portion 77 has a left handthread. The healing screw may be provided in a range of sizes, but ineach case the height of the head portion is of the order of 1 mm orless.

The extension or rest factor member 18 as it appears prior toinstallation in the jaw is illustrated in FIG. 2. The member comprises agenerally cylindrical boss 36 having a projecting shaft 38 at one enddimensioned for mating engagement in the bore 32 of implant 12. Shaft 38has an upper, larger diameter portion 39 having external screw threadsfor threaded engagement with the internal screw threads 34 in the upperpart 33 of bore 32, and a lower, smaller diameter cylindrical extension40 for fitting into the lower part 35 of bore 32, as illustrated in FIG.11. The upper end of shaft 38 has a taper 41 matching the taper 37 atthe upper end of the implant bore 32. Member 18 has a curved, slightlyconvex rest or support surface 42 at its upper end, and an external hexformation 43 for securing it to a suitable tool for insertion into theimplant. The member 18 will also be of a suitable dental material suchas titanium alloy.

Member 18 may alternatively be formed with a concave support surface(not illustrated), or with any suitably shaped upper rest or supportsurface. Member 18 will be made in a range of heights to allow thedentist to select the appropriate size rest factor for a particularpatient's tissue level. The rest factor is selected to project only to adistance of the order of ½ to 1 mm above the tissue level when installedin the jaw, and is preferably kept as low as possible so that it acts toaccept biting pressures but will not interfere with normal jaw motions.

The implant assembly with the rest surface 42 is used to form a rest orsupport surface in implant dentistry for a prosthesis anchored elsewherein the jaw, for example, as illustrated in FIGS. 12 and 13. The implantassembly is designed to be relatively short or thin so that it can beinstalled in distal jaw regions to the rear of line 160 in FIGS. 12 and13, which extends between the first and second bicuspids 161 and 162 andcorresponds to the approximate location where the mandibular nerve 163exits the jawbone. The mandibular nerve extends through the jawbone tothe rear of the first bicuspid, making the installation of long implantsin this region difficult or impossible without the assistance of adental surgeon. Thus, unsecured dentures are common, resulting in boneerosion from biting impacts. This rest factor assembly avoids or reducessuch problems. The implant has a relatively large diameter as comparedto its height, providing good implant to bone contact and integration,while having only a short penetration into the jawbone. In the preferredembodiment illustrated, the penetration into jawbone is only between 2to 4.5 mm, depending on the patient's bone depth, and there is thus norisk of interference with the mandibular nerve.

FIGS. 12 and 13 illustrate the assembly implanted in the jaw forcooperation with an overlying denture anchored elsewhere. In FIGS. 12and 13, a full denture or prosthesis 164 is illustrated, secured to apatient's lower jaw 165 via a pair of anchors 166, 167 mounted in theanterior regions of the patient's jaw, and seated on implanted restfactors 168, 170 in the posterior or distal jaw region below the formersecond molar at each end of the denture. In practice, the rest factorassembly may be installed anywhere between the region corresponding tothe former second molar (line 171) and the first bicuspid (line 160).The area to the rear of the line 171 has too many muscles for insertionof an implant. As illustrated in FIG. 13, the rest factors may beprovided or preformed with a slightly convex or ball-shaped head 42which projects from around ½ to 1 mm above the tissue level, althoughsupport surfaces of a different shape may be provided. The prosthesis ordenture is preferably provided with a corresponding concave surface ordepression for seating on the opposing rest factors or surfaces 168 and170.

Most typical denture anchors allow a limited degree of pivoting orside-to-side motions of the denture with jaw motions so as to reducestress in the jaw bone areas to which the denture is positivelyanchored. With relatively long partial or full dentures which extendinto the posterior jaw regions, tilting or rotation of the denture aboutthe anchor points with jaw or biting motions will apply pressure orbiting force to the underlying tissue and jawbone, causing discomfortand trauma to the underlying bone and tissue, and ultimately resultingin significant bone erosion. The rest surface or surfaces avoid orreduce this problem by accepting down or biting pressure from thedenture. Since the rest surfaces are not positively anchored to thedenture, they will not be pulled up or from side to side as a result ofjaw motions, and will therefore be less likely to cause bone erosion ordamage. The implant assembly is therefore intended to be used atappropriate locations in the jaw which would otherwise be subject toconsiderable down pressure and potential trauma from an implant such asa full or partial denture or cantilevered bridge. The rest factorsurface will be shaped to ensure that it can accept down pressure froman overlying denture in various possible orientations, whatever theangle of the patient's jaw. The support surface prevents excessive forceon the tissue, and thus protects the gum tissue from impacting forceswhich could cause soreness and trauma. Additionally, the rest or supportsurface prevents or reduces bone erosion by providing support to thedistal end of the denture and resisting biting forces.

The implant assembly is designed for implantation at any position in thejaw where a rest factor or surface for a full or partial denture orbridge is needed. Normally, this will be in the posterior jaw, forexample in between the first bicuspid and the second molar area, asillustrated in FIGS. 12 and 13, but a rest surface may also beadvantageous in other areas. The dimensions of the assembly 10 areselected according to the dimensions of the jaw in the area where theimplant is to be used. A range of implant assemblies of differentdimensions may be provided for fitting patients having different jawdimensions. The implant member is designed to provide sufficientanchoring area with the jawbone when embedded in the jaw, while notextending too deeply into the jaw where it might otherwise interferewith the nerve. The implant member has a relatively large diameter,larger than typical straight cylindrical implants, but is shorter inlength than such implants, preferably having an overall length in therange from 2 mm to 4.5 mm. In one specific example, several differentsize implants were provided with annular members of 4.25 mm, 5 mm, and 6mm outer diameter, respectively. The rest factor may have a diameter of4 mm. The extension or spigot 28 may also be provided in different sizesaccording to the position in the jaw where it is to be embedded. Centralspigots of length between 1 mm and 3 mm may be provided. Also, thecentral spigot may be omitted in some cases where very little depth isavailable in the jawbone for embedding the implant. In this case, therest factor 18 will also have no projection 40. Rest factors havingheads in a range of different sizes are also provided, for example, 3mm, 4 mm and 5 mm. Generally, the overall implant assembly is veryshort, extending only from 2 to 4.5 mm into the jawbone, while theimplant is shaped to have a relatively large bone to implant contactarea, as best illustrated in FIG. 5.

As can be seen in FIG. 5, after osseointegration has taken place, therewill be a relatively large area of bone to implant integration eventhough the penetration into the jaw bone is very short. The implant hasfirst, circumferential surfaces 180, 182 which resist sideways movementsof the dentures, while the second, lower surfaces 184, 185, 186 of theouter rim 30, upper face, and boss 28 resist vertical movements. The twosurfaces combine to provide maximum rest factor to bone contact with arelatively short distance of penetration into the jawbone. The grooves29 in the outer surface will resist rotational movements of the implant.Thus, the implant surfaces resist lateral and vertical looseningmovements, and the implant effectively becomes fully integrated with thebone.

This implant is sufficiently short to be safely inserted even where somebone erosion has already taken place, with the appropriate heightimplant member and rest factor being selected according to the bonedepth and tissue height of the particular patient.

The method of inserting the implant 12 in the jaw will now be describedwith reference to FIGS. 6-11 of the drawings. This can easily be done bya dentist or dental surgeon. First, a bore shaped to correspond to theshape of member 12 must be cut out. This is done using a series ofspecial cutting burrs. A first water cooled burr or cutter (notillustrated in the drawings) is used to drill a cylindrical guide holeor pilot dimple at the center of a selected site, for example under thesecond molar area or at the distal end of a cantilevered bridge. Thewidth of the alveolar crest with equal distance on both sides of thepilot dimple is then measured. The largest diameter rest factor implantwhich will fit within the available width while allowing at least ½ mmof bone on each side of the implant is selected. An internally irrigatedimplant body drill 44 of diameter matching that of the selected restfactor implant is then selected. Burr 44, illustrated in FIG. 6, has asmaller diameter pilot drill 46 for cutting out a cylindrical bore 48and a larger diameter portion 50 having an end cutter 52 for drillingout the larger diameter upper end 56 of the bore. The cylindricalportion 50 may be provided with suitable markings or a scale (not shown)so that the dentist can control the depth the drilled bore. However, inthe preferred version, the height of portion 50 matches the height ofimplant member 12. The length of the pilot drill 46 will correspond tothe length of spigot 28 of the implant, so that spigot 28 will fit inbore portion 48. The dentist determines the optimum angle and drills into the bone to a point where the larger diameter portion 50 ends.

FIG. 7 illustrates the operation of a water cooled guided core drill orburr 58 having a central guide or pilot tip 59 for fitting in thepreviously drilled bore portion 48 to center the tool on the bore. Thetool has a cylindrical central body portion 60 having a downwardlyfacing annular ring of cutting teeth 62 for drilling out an annulargroove or channel 64 around the periphery of the flat or shoulder 66separating counter bore 56 from the smaller diameter bore portion 48.The length of the teeth controls the depth of groove 64, and will beequivalent to the height of the downwardly depending rim 30 of theimplant to be received in the bore. Once the lower face of body portion60 hits the flat 66, drilling is stopped.

All three cutting tools may be provided with a cage for collecting boneas it is drilled out of the bore. The collected bone may be used forfilling any edges or spaces left after insertion of the implant.

After the bore for receiving the implant 12 has been prepared asdescribed above, and debris has been cleaned out in the standard manner,for example by irrigating the site with sterile water or sterile saline,the implant 12 can be inserted. The healing screw 70 is initiallysecured to the implant.

Prior to insertion in the previously prepared bore, the internal facesof the implant will be coated with a suitable bonding agent, such ashydroxyl apatite. These faces may be roughened as illustrated in FIGS. 1and 3 to increase the bonding area and strengthen the adhesion in thebore 14. A suitable plastic insertion tool 67 (see FIG. 1) having ahandle and a gripping end for snap engagement over the head of thehealing screw is then used to insert the implant and attached healingscrew in the bore. The bore is drilled out to a depth such that theupper end of the implant 12 will be at the bone level when fullyinserted, as indicated in FIG. 8, or slightly below that level ifdesired. For convenience, the insertion tool is preferably a disposable,snap-off plastic member which is supplied in a sealed, sterile packagetogether with the healing screw and implant, the three parts beingsupplied secured together in the package for easy handling. Theinsertion tool can be snapped off after the parts have been positionedin the bore.

The tissue or gum 21 is then secured over the implant and healing screwwith conventional flap sutures 80. Since the head portion of the healingscrew projecting above the implant member is relatively short, little orno bulge will be apparent when the tissue is sewn up. The site is leftto heal for several months to allow the implant to osseointegrate, orbond with the surrounding bone. At this time, a special locating tool82, best illustrated in FIGS. 4A, 9 and 10, is used to locate theimplant.

The retrieval tool 82 comprises a central shaft 84 with a head orgripping handle 86 at one end. Handle 86 projects to one side of theshaft as indicated. A projecting probe 88 at the opposite end isdesigned for engagement in the bore 76 of healing screw. The probe has asharp pointed end 91. A cutter sleeve or tissue punch 92 is slidablymounted on shaft 84. Cutter sleeve 92 has a projecting annular handle orgripping portion 94 and a lower cutting edge 96.

The use of the retrieval tool 82 to locate the implant site will now beexplained, with reference to FIGS. 1 and 9. The approximate area of theimplant is first located, utilizing radiographic charts and fingerpalpations. The pointed end 91 of the probe is then used to pierce thetissue 21 at the approximate site of the implant, and is then used as aprobe to locate the upper face of the healing screw. Once the screw hasbeen found, the concave recess (if provided) will act as a guide todirect the pointed end to the retrieval hole or bore 76 at the center ofthe healing screw, as illustrated in FIG. 9, centering the tool on theimplant site.

Once the probe has entered bore 76, as illustrated in FIG. 9, the handle86 of the locating tool is held firmly in one hand to support the toolupright and the tissue punch is turned in a circular motion whilepushing it down along the locating tool with a firm pressure. The tissuepunch is designed to cut out a plug 97 of tissue directly over theimplant. The punch will be stopped when it engages the outer diameter ofthe healing screw. The locating retrieval tool 82 is then removedtogether with the tissue punch, simultaneously pulling out the plug oftissue. If the tissue plug does not pull out, it may be removed withforceps.

A separate healing screw removal tool 110 is then used to remove theexposed healing screw. Tool 110 is illustrated in FIG. 4B and 11, andcomprises a shaft portion 112 with a head or gripping portion 114 at oneend and a threaded portion 116 at the opposite end for threadedengagement in the threaded, upper end portion 77 of bore 76 of thehealing screw. The threaded end 116 is threaded counter-clockwise intothe healing screw, tightening the tool inside the healing screw and atthe same time unscrewing the healing screw from the implant asillustrated in FIG. 11. The implant is thus exposed for secondaryhealing or restoration procedures.

This technique for exposing or recovery of an embedded implant afterhealing and osseointegration has taken place removes only a small plugof tissue from immediately above the implant site, avoiding the need tocut out an enlarged flap of the tissue both to locate the implant and toexpose the healing screw for removal. Little or no suturing will berequired. Thus, considerably less trauma to the tissue is involved,reducing the healing time and the risk of infection. Also, the healingscrew is located and the tissue plug may be removed simultaneously withone tool, simplifying the procedure and reducing the time involved. Themethod involves the use of a specialized healing screw with a bore inits upper surface, together with a special locating and retrieval tool.It may be used not only for location of the implant 12 as describedabove, but also for locating any conventional cylindrical implants inimplant dentistry, replacing the conventional healing screws of suchimplants with a healing screw as illustrated in FIG. 1 but having a stemor shaft designed for fitting into the implant bore. Although in thepreferred embodiment described above, the upper end of the retrievalbore 76 in the upper face of the healing screw is screw threaded, it mayalternatively be hexagonal with the retrieval tool having acorresponding hexagonal portion for mating engagement in the bore.

As an additional aid in locating the embedded implant, a thread or wiremay be left projecting from the heading screw through the suture area,so that the location may be found easily after healing. Alternatively,the tissue overlying the implant may be marked with a suitable dye.However, it is expected that such markers will not normally be required,the dentist locating the general implant site by feel before piercingthe tissue with the probe.

The same implant recovery tools may be used for any selected implantsize, since the dimensions of bore 76 in the healing screw will beidentical.

Once the healing screw has been removed and the exposed surface of theimplant suitably cleaned and prepared, the appropriate rest factor 18 isinserted into the implant. The rest factor 18, in addition to providinga rest surface, also acts as a secondary tissue healing insert. The restfactor is selected with a head height so that it will project just abovethe patient's tissue level when installed. The shaft 38 of rest factor18 is screwed into bore 32 of implant 12, as indicated in FIG. 5, withthe mating surfaces first being coated with a suitable bonding agent.

Although the rest factor in the preferred embodiment has a head portionpreformed in a range of heights, it may alternatively be provided with alonger extension piece which projects above the tissue level 20 when themember 18 is fully inserted. In this case, the dentist marks around theperiphery of the selected member 18 at the tissue height, and removesthe member from the implant. A suitable temporary cover or crown of astandard nature may be fitted into implant 12 at this point.

The dentist then mounts the member 18 in a previously prepared cast ofthe patient's jaw, and machines or cuts away the upper face of member 18to provide the desired rest surface 22 at the tissue level 20, asdetermined by the markings made while the member was mounted in thepatient's jaw. The cut away surface may be slanted or inclined accordingto the angle of the patient's tissue or gum. This allows the height tobe customized for minimal side torque. The shape of the rest surface 22may be of the dentists choice. For example, it may be concave, while thedenture or prosthesis with which it is to cooperate has a correspondingconvex area or bump 95 for fitting into the concave depression on therest surface, so that the rest factor or member 18 accepts down pressurefrom the denture without any retention. However, in the preferredembodiment, members 18 with ready-made ball-shaped or other shape headsof various sizes in a range of tissue heights are provided to avoid theneed for machining on site by the dentist.

This procedure may be utilized to implant one or more rest factors atany suitable location in the jaw, depending on the denture pressurepoints, for example as illustrated in FIGS. 12 and 13.

FIG. 14 of the drawings illustrates an alternative embodiment of theimplant assembly which is much thinner than that of FIGS. 1-3 and 5 andwhich will therefore project only a minimal distance into the jawbone,further reducing the risk of interference with the nerve. This implantassembly is useful for providing a rest surface at a desired location inany patient's jaw, whether or not previous bone erosion is a factor, butis particularly useful in patients having significant bone erosion wherevery little depth is available for implants.

FIG. 14 illustrates an implant member 180 and healing screw 182 of animplant assembly according to a second, modified embodiment of theinvention. FIG. 18 illustrates the implant member 180 and healing screw182 of FIG. 14 implanted in the jawbone during the osseointegrationprocess, while FIGS. 15-17 illustrate a modified method of forming abore in the jawbone for receiving the implant.

As illustrated in FIGS. 14 and 18, implant member 180 is a flatdisc-like member having an undersurface of similar shape to theundersurface of member 12 in the first embodiment. However, theperipheral rim 184 and central spigot 186 are approximately the samelength in this embodiment, so that the spigot 186 does not projectdownwardly below the lower end of rim 184. Preferably, member 180 has atotal height of around 2 mm while its peripheral rim 184 projects around1 mm below the undersurface of disc part 188. The member 180 is providedin a range of diameters, preferably 4.25 mm, 5 mm and 6 mm, for patientshaving varying alveolar ridge widths. As in the first embodiment, themaximum diameter possible implant member is selected for the patientdependent on the available space, i.e., the alveolar ridge width. Therim 184 is relatively thin, and in one particular example had athickness of the order of 0.4 mm.

The member 180 has a recess 190 in its upper surface with a taper 191extending around the outer periphery of the recess. A central, straightcylindrical bore 192 extends from the center of recessed area 190 intothe spigot 186, and bore 192 has screw threads 193 extending along itslength. Member 180 is made of the same material as the implant 12 of thefirst embodiment. As in the first embodiment, circular or roundedindents 194 are provided on the outer surface of member 180 to resistrotational movement after implantation. Between six and eight equallyspaced indents may be provided, for example.

Healing screw 182 has a relatively short head portion 195 and adownwardly depending, screw threaded shaft portion 196 for matingengagement in the bore 192 of implant member 180, as illustrated in FIG.18. The undersurface of head portion 195 seats in recessed area 190 andhas a tapered annular surface portion 197 for seating on taper 191around the recessed area 190, for accurate seating of the screw in bore192. The upper surface of head 195 has a central, tool receiving bore198 for receiving the end of a suitable tool for inserting thearrangement in a previously prepared bore in the jawbone, and also forreceiving the end of locating tool 82 as described above in connectionwith the first embodiment of the invention. Bore 198 is of hexagonalcross-section, and is designed to be removed by a suitable removal toolhaving a hexagonal end after location by tool 82.

The rest factor or member of the second embodiment is not illustrated inthe drawings but will be similar or equivalent to rest factor 36 asillustrated in FIGS. 2 and 5 of the drawings apart from its lowersurface and downwardly depending shaft portion, which will be identicalto lower surface and shaft portion of the healing screw 182 for matingengagement in the bore 192 in implant member 180 after osseointegrationis complete.

The modified method of inserting insert member 180 in the jaw will nowbe described with reference to FIGS. 15-17 of the drawings. Thisprocedure can easily be carried out by a dentist, although a dentalsurgeon may also perform the procedure if desired. After the tissueoverlying the implant site has been cut, a pilot dimple is formed at thecenter of the selected site. The width of the alveolar crest or ridge atthe implant site is measured, and the largest possible diameter implantwhich will fit within the available width while leaving at least ½ mm ofbone on each side is selected.

A bore matching the selected implant dimensions is then accuratelydrilled out using a series of three internally irrigated drilling burrs.The first burr 210 has a straight pilot drill 212 for drilling out acylindrical bore 214 to a desired depth at the implant site, asdetermined by stop 216, as illustrated in FIG. 15. Preferably, bore 214will be slightly longer than the implant member, for example 3 mm. Asecond burr 218 is designed to cut out the desired bore shape to matchthe shape of the undersurface of implant member 180, as illustrated inFIG. 16. Burr 218 has a central, non-cutting guide or spigot 220 forfitting into previously drilled bore 214 for centering purposes, a firstcutting surface 222 for cutting down to the level of flat 224, and anannular, downwardly projecting rim of cutting teeth 226 for cutting outpart of annular recess 228 for receiving the annular rim 184 of theimplant member. Preferably, teeth 226 are designed to cut recess 228 toa depth of ½ mm. The final burr 230 is illustrated in FIG. 17 and isdesigned to finish and smooth the surfaces of recess 228. Burr 230 alsohas a central guide 231 and an annular rim of finer cutting teeth 232which cut the final ½ mm of the recess to a total depth of around 1 mm,and which smooth and finish the cut surfaces.

The finished bore of FIG. 17 is cut to very precise dimensions by theseries of cutting drills so that the implant member can be accuratelyseated in the bore as illustrated in FIG. 18 after suitable treatment ofthe surfaces and application of bonding agents. The gap below spigot 186does not affect the integration process and will soon fill in with bone.The accurate, close fitting of peripheral rim 184 into recess 228provides great stability and resistance against any sideways movementduring the three-month or more osseointegration period, so that a goodbone-to-implant bond can be produced in spite of the minimal length ofthe implant.

After osseointegration is complete, the site is located and the healingscrew 182 exposed and removed as described above in connection with thefirst embodiment of the invention. The rest factor (not illustrated) isinserted into the implant member as described in connection with theprevious embodiment.

In both of the embodiments described above, the shape of theundersurface of the implant ensures that there will be little or no sidesway either during or after the osseointegration period. This resultsfrom the downwardly projecting peripheral rim, having inner and outercircumferential surfaces which combine to resist any sideways forces.This resistance to side-sway is enhanced by the spigot 186 which alsoacts to resist sideways movement. The relatively large diameter of theimplant provides a large area of downwardly facing surfaces whichtogether resist downward forces on the implant assembly, furtherincreasing the stability of the implant and acting to absorb bitingpressures. The implant is selected to be of the maximum possiblediameter according to the bone width available for implantation in aparticular patient. The indents 29, 194 will act to resist rotationalmovements during and after osseointegration. The combined effect of theshape of the undersurface of the implant and its relatively largesurface area is to produce a very stable implant with minimalpenetration into the bone.

Although the implant member is illustrated as implanted so that itsupper surface is at the bone level, it may be implanted to a lesserdepth if the patient has a large amount of bone erosion or resorption.For example, if there is only 1 mm bone depth available for implantationwithout fear of interference with the nerve, the implant is simplyinstalled to 1 mm in depth so that approximately 1 mm projects above thebone level. However, it will still have sufficient holding power toremain in position since the undersurface, and particularly theperipheral rim, will position the implant during osseointegration andbond to the surrounding bone to resist sideways and downwards forces.Since it is not anchored to any overlying body, upwards forces do nothave to be resisted. Thus, sufficient bonding area is provided to resistany loosening during normal wear.

This implant system and method may be used in any implant procedurewhere a denture or prosthesis of more than one tooth is involved, and isparticularly useful in posterior areas of the jaw where the implantdepth is limited, for example the second molar area, and in conjunctionwith anterior implants or cantilevered bridges. The implant has arelatively short penetration into the bone, so that it can be installedin regions to the rear of the first bicuspid without fear ofinterference with the nerve, yet has sufficient anchoring surface areato integrate with the bone and accept down pressure of two to threehundred pounds from an overlying denture or prosthesis. Since theimplant is not positively anchored to the prosthesis, it does not haveto resist large upward or sideways forces, reducing the risk of boneerosion. At the same time, the rest factor will reduce the trauma tounderlying tissue and reduce or eliminate bone erosion from theoverlying denture by accepting the downward pressure from the denture.

The implant recovery method and tool described above will eliminate theneed to locate and expose osseointegrated implants by a surgical flaptechnique. The locating tool and guided tissue punch accurately locatethe implant with minimal trauma, and remove only a plug of tissuedirectly above the implant sufficient to expose the healing screw forremoval. The amount of trauma and bleeding is reduced and the tissuearound the implant site remains virtually intact.

FIG. 19 illustrates a rest factor 250 according to another embodiment ofthe invention. The rest factor 250 is generally cylindrical and has anupper, dome-shaped portion 252 and a downwardly depending, threaded stemportion 254. The dome portion 252 has hex bore 256 formed in its upperend for gripping by a hex tool on installing or removing the rest factor250, as will be explained in more detail below. The hex bore will berelatively small, preferably of the order of 0.050″, and may be filledwith a temporary sealant after installation.

The rest factor 250 is made from a suitable metal such as titaniumalloy. The dome portion has a wear resistant titanium nitride coatingapplied. Rest factors are provided in a range of different dimensions.The dome portion is preferably provided in a range of different heights,and in a preferred embodiment rest factors were provided with domeportions of height 1.5 mm, 2.5 mm and 3.5 mm for fitting patients withdifferent gum thicknesses. The total height of the rest factor ispreferably in the range 2 mm to 4.5 mm.

FIGS. 20, 21 and 22 illustrate three alternative types of implant forimplanting in the jaw bone at a selected site to receive rest factor 250or other denture fitting devices Such as denture anchors or magneticabutments of a conventional type. The implant 260 of FIG. 20 isrelatively short and has a body of generally cylindrical shape having anupwardly tapering upper end portion 262 and a downwardly depending stemportion 264. A threaded bore 266 extends downwardly from the upper endof the implant for receiving the threaded stem portion of the restfactor 250. An eight-sided cut 268 extends through the threads, asillustrated in FIGS. 20 and 23. The outer diameter of the implant at itsupper end 270 is dimensioned to match the outer diameter of the domeportion of the implant at its lower end 272.

The stem portion 264 of the implant 260 has an annular recess 274extending upwardly from its lower end to a position close to but offsetdownwardly from the lower end of the recess 268. Recess 274 forms anouter annular rim 276 and a central boss 278 at the lower end of theimplant, and provides an area for bone growth and osseointegrationupwardly into the implant. Thus, the implant is generally cup-shaped atits lower end, with a central protrusion in the cupped area. A triplelead screw having a first lead or thread 279, a second thread 280, and athird lead or thread 282 is formed around the outside of the stemportion 264. The threads 279, 280, 282 are preferably of squarecross-section and project out a predetermined distance from the surfaceof the stem portion 264. The threads may each extend aroundapproximately 180° and preferably do not overlap, although they mayoverlap in alternative embodiments.

FIG. 21 illustrates another implant 284 which is longer than that ofFIG. 20. Again, the implant is generally cylindrical and has arelatively thin upper annular portion 286 with a short, upwardlyprojecting and tapered rim 288 and a downwardly depending stem portion290 of reduced diameter. A bore 292 projects downwardly from the upperend of the implant 284 and has a threaded portion 294 for receiving thethreaded stem of a rest factor member. A ten-sided or decagon cut 296 isformed through the threads, and preferably extends about halfway downthe bore 292. In this version, the bore 292 projects downwardly into thestem portion of the implant, since the upper portion 286 and rim 288 areshorter than those of implant 260.

A triple lead thread having first, second and third threads 297, 298 and299 is formed around the outer surface of stem portion 290. Preferably,the threads 297, 298 and 299 start at equally spaced intervals aroundthe periphery of the stem portion adjacent its upper end but spaceddownwardly a distance below the annular ring portion 286. The threadsextend around the stem portion towards its lower end and terminate at alocation spaced above the lower end of the stem portion. As in theversion of FIG. 20, the stem portion 290 has an annular recess 300 atits lower end forming an annular rim 302 and central boss 304. As in theprevious embodiment, the upper end 306 of the implant is of diametermatching that of the lower end of the rest factor.

FIG. 22 illustrates another, longer implant 310 which is similar to thatof FIG. 21, apart from the fact that the stem portion 312 is longer anda quadruple lead screw is provided on the stem portion. The implant ofFIG. 22 is otherwise identical to that of FIG. 21 and like referencenumerals have been used for like parts. The implant has a bore identicalto that of implant 284 extending downwardly from its upper end, and anannular recess at its lower end which is identical to the recess ofimplant 284. As in the previous two embodiments, the upper end 304 ofthe implant has a diameter matching that of the lower end of the domeportion of the rest factor.

A series of four separate threads 314, 316, 318 and 320 is providedaround the periphery of the stem portion 312. The threads start atequally spaced intervals around the periphery of the stem at a locationclose to but spaced slightly below the upper end of the stem portion, toleave a gap between the annular ring 286 and the threads. The gap is inthe range from 1 mm to 2 mm.

Although in the illustrated embodiment the implants 260 and 284 eachhave three threads, and the longest implant has four external threads,any one of these implants may have two, three or four threads. Thedouble, triple or quadruple lead thread allows for quick and solidengagement into a tapped bone site. The projecting screw threads form anundercut region and bone growth and attachment in this area increasesthe strength of the attachment and resistance to loosening forces. Theunique cup shaped recess design at the lower end of the implant is alsodesigned to increase bone to implant surface contact and attachmentarea.

The pitch or angle of the thread is preferably relatively steep and ispreferably in the range from around 9° to 17°. The spacing betweenadjacent threads is preferably at least 1 mm. It has been found thatthis is the minimum spacing required to ensure significant bone growthand osseointegration in the gaps between threads. The threads preferablyproject out around ½ mm from the surface of the stem portion of theimplant, and they are about ½ mm in height. Thus, the gaps between thethreads are twice as wide as the threads themselves. This provides goodblood supply to the bone between the threads, and provides a greaterbone thickness and stronger shelf of bone between the threads.

Both the implants and the rest factor are all made in a range of lengthsand diameters to fit the size of different patient's jaws. Implants willbe provided in a range of lengths. Preferably, the lengths are of theorder of 2 mm, 4.5 mm, 7 mm, 10 mm and 13 mm. The 2 mm length implantwill be of the type illustrated in FIG. 20, while the 4.5 mm implantwill be of the type illustrated in FIG. 21 and the 7 mm, 10 mm and 13 mmimplants will be as illustrated in FIG. 22. Each of the differentimplants will be provided in a range of different outer diameters,preferably 4 mm, 5 mm and 6 mm. Preferably, the height of the taperedrim of the implant 260 or 310 is as short as possible, for examplearound 0.5 mm.

The implants are made of a suitable material such as titanium and may becoated with a coating of a material for improving bone adhesion.

The method of installing implants 260, 284 and 286 will be similar withappropriate changes in the dimensions of the drilled bore to accommodatethe different length stem portions and numbers of threads. The methodwill therefore be described for the implant 260 only, by way of example.The implantation method is similar to that illustrated in FIGS. 6-11 anddescribed above.

The first step in the implant procedure is to select the site in the jawat which the implant is to be installed. The appropriate height anddiameter implant member is selected dependent on the bone depth, tissueheight, and jaw thickness at the selected site. The minimum bone depthrequired is 1 mm between the bottom end of the installed implant and themandibular nerve canal or maxillary sinus space. Thus, the total bonedepth required for installing the 2.5 mm implant is only 3.5 mm, whilethe longer implants can be installed where a greater bone depth isavailable. The minimum bone width required to place a rest factorimplant is ½ mm each on the lingual and facial sides of the implant,requiring a 5.0 mm total ridge width for the 4.0 diameter implant, 6.0mm ridge width for the 5.0 mm diameter implant, and 7.0 mm ridge widthfor the 6.0 mm implant. Based on these dimensions, the appropriateimplant can be selected.

The very short, 2.5 mm implant can be installed safely even where somebone erosion has already taken place and even in the distal jaw regionswithout risking interference with the mandibular nerve. The design issuch that the implant has a relatively large surface area for bone toimplant osseointegration, and undercut areas for improved retention andresistance to movement in any direction.

Once the appropriate implant has been selected, a bore shaped anddimensioned to correspond to the shape and dimensions of the implant iscut out. This is done in a series of steps. First, a mesio-distalincision is made through the tissue or gum 321 along the alveolarmid-crest at the selected site, typically the area of the second molar.A bone plateau is created which is made as flat as possible by removingridges or other bone irregularities. At this point, the width of thealveolar crest can be measured to determine the largest diameter restfactor implant which can be fitted within the available width and stillallow at least ½ mm of bone on each side. The implant is placed as closeas possible to the lingual side while still allowing the ½ mm of bone onthe lingual side.

At this point, a pilot burr is used to drill out a pilot hole ofappropriate depth at the center of the insertion site, as illustrated inFIG. 15 above. The pilot hole diameter will be substantially identicalto the diameter of the implant central boss 278. An appropriate depthlimiting sleeve is used with the pilot burr in order to limit the depthof the pilot hole to match the implant height. Thus, depth limitingsleeves will be provided for each of the four different implant heights,and the appropriate sleeve will be selected on installation. The lowerend of a previously drilled pilot hole 322 in jawbone 323 can be seen inFIG. 24, which illustrates a subsequent step in the procedure.

An alignment pin may be placed in the pilot hole to check for properalignment to the path of a prosthetic insertion and between multipleimplants.

After alignment, a spot-face drill (not illustrated) is used to make ashallow, 360° shoulder or seat 324 into the crest of the bone. This seatis used to assist in engaging of a bone tap in a later stage of theprocedure. The drill is withdrawn several times during site preparationto remove bone buildup between the flutes of the drill. An implant bodydrill is used to drill out the main recess 328 for receiving theimplant. This drill will be similar to drill 44 illustrated in FIG. 6,above, and will have a diameter corresponding to that of the stemportion of the selected implant. Thus, the drill will have a diameterequal to the implant diameter less the thickness of the threads, or 4.0mm in the case of a 5.0 mm implant, for example.

In the next step of the procedure, a core drill 326 is used to cut anannular ring or recess 330 at the bottom of recess 328 for receiving theunderside rim 272 of the implant. Drill 326 has a central guide 332 forcentering in the previously drilled pilot hole 322, a cylindricalcentral body portion, and a downwardly facing annular ring of cuttingteeth 334 for drilling out an annular groove or recess 330 for receivingthe rim 272 (or 302 in the case of the implant 284 or 310) of theimplant.

A series of three guided hand taps (starting tap, intermediate tap andfinish tap) are used to make a multiple lead tapped preparation into thedrilled hole 328. It will be understood that the triple and quadrupleleads will be made in a similar manner. One of the thread taps 336 isillustrated in FIG. 25. Each tap has a tap guide pin 338 which engagesthe pilot bore to keep the tap centered on the recess. Optionally, a tapguide template (not illustrated) may be secured across the recess viapins secured on opposite sides of the implant site. This can be used toprovide better support for the bone tap if the patient has spongy,cancellous bone. The tap 336 has triple lead cutting flutes 340 forforming a thread matching that on the insert. The starting tap has amachined line indicating the start of the double cutting edge of thebone tap. The starting tap is positioned into the drilled recess and thecutting flutes are engaged into the cortical bone, with the machinedline on the tap positioned perpendicular to the longitudinal axis of theridge. The implant site is then hand tapped and threads 342, 343 and 345are tapped into the recess. The tap is backed off a quarter of a turnseveral times during the process to clear bone chips. In this way, thetap is worked down to the bottom of the drilled recess 328.

The procedure is then repeated using an intermediate tap, which will bemarked with two lines to distinguish from the other two thread cuttingtaps. The lines are positioned in the same starting position as the lineon the starting tap. Finally, the procedure is repeated with the finishtap, which will be marked with three lines, to create a final cut of thebone thread to accept the implant.

An octagonal wrench 344 is used to place the implant into the preparedimplant recess, as illustrated in FIG. 26. The octagonal end of wrench344 will engage in the octagon 268 cut through the threads 266 and willnot damage the threads in bore 266. The implant is completely seatedwith the implant threads completely engaged in the bone threads and thetapered upper portion of the implant projecting above the level of thebone, as illustrated in FIG. 26. If the implant is set in correctly, thestem portion 264 will have its upper end located ½ to 1 mm below the topof the bone, as illustrated in FIG. 26.

A healing screw is then threaded into the implant to protect theinternal threads of the implant during the osseointegration period. Thehealing screw is similar to the healing screw 70 of the first embodimentbut with a threaded stem matching bore 266 and a hex hole in place ofthreaded hole 76 at the upper end of the screw. The flap of gum 321 issewn down over the implant site, in a similar manner to that illustratedin FIG. 8 above, after installation of the healing screw.

The implant is then left for a healing period to allow the bone toosseointegrate with the surface of the implant. A healing period of notless than 4½ months should be used. The bone will grow over andosseointegrate into the gaps between the threads, providing a greaterosseointegration surface area and better retention of the implant. Thecup-shaped design of the lower end of the implant, along with themultiple lead threads, increases bone to surface contact and helps towithstand posterior occlusal load.

Where a triple or quadruple lead thread is used, resistance to rockingor movement of the implant is provided. With two opposing threads, theremay be some tendency for the implant to rock from side to side. Thus,implants with three or four threads are used for even greater resistanceto rocking as a result of loads.

After the minimum period required for osseointegration, an incision ismade over the implant for access to the healing screw. Radiographiccharts and finger palpation may be used to locate the general area ofthe implant. The incision may be made in a conventional manner, butpreferably a guided tissue punch 82 is used as in the previousembodiments. The tissue is pierced with the sharp point or probe 91 ofthe tissue punch, and the implant site is probed until the point 91locates a hex hole provided on the top of the healing screw. The guidedtissue punch 92 is then turned down on the guide with firm pressureuntil it punches an opening through the tissue or gum. The tissue punchwill engage the outside diameter of the healing screw, and will cut outa plug of tissue directly over the center of the implant, as illustratedin FIG. 10 above.

The locating tool/tissue punch assembly is then removed, simultaneouslypulling out the cut plug of tissue. If the tissue does not pull out withthe tissue punch, forceps may be used to remove it and expose thehealing screw. A cover screw hex tool (not illustrated) is used toengage the hex hole at the top of the cover screw and remove the coverscrew from the implant.

The implant is now exposed for attachment of the dome-shaped rest factor250 as illustrated in FIG. 27. The implant may alternatively be attachedto other dental devices, such as implant anchors or magnetic abutmentsfor dentures. The dimensions of the selected rest factor will depend onthe implant dimensions and also the tissue height of the patient. Theheight of the implant should be such that the top of the dome is at thetissue level or only slightly above it. The dome portion of the restfactor selected should therefore have a height substantially equal tothe tissue or gum thickness. If the gingival layer has a heightsubstantially greater than the largest dome portion, tissue reductionmust be used to reduce the gingiva thickness to a maximum of 3 mm at theimplant site.

Once the correct size rest factor has been selected, a hex wrench 346having a hexagonal shaft 348 is used to engage the hex bore 256 at theupper end of the dome-shaped upper end portion 252 of the rest factor.The rest factor is then threaded into the bore of the implant, asillustrated in FIG. 27. The tissue is then sewn back around the implantdome, as indicated in dotted lines in FIG. 27, and the dome is left inplace for around 4-6 weeks to allow for a period of reduced implantloading during gingival healing. The patient's denture should be fittedto the dome top with a soft reline material to maintain proper occlusionduring the gingival healing period. This is done by taking a full archreline impression including the exposed metal tip of the rest followedto prepare a master cast of the jaw including the exposed tip of thedome. The denture base is relined using this cast in the normal manner.

If a change in tissue height has occurred during the tissue healingperiod, the rest factor may be unthreaded and replaced with another restfactor of the correct dimensions.

Once the healing period is complete, the top of the dome is exposed fora prosthesis to rest on with lateral freedom of movement. The preferredarrangement is to use the dome as a permanent, non-retentive rest whenthe patient has existing anterior retention for a prosthesis. However,the rest factor may be used as an attachment if required, for example ifthere is no existing retentive means for the prosthesis, or lateralstability is needed due to severely reduced ridge height. A hole isdrilled into the center of the dome to an appropriate depth, and resinis injected into the counter-bore. When the resin starts to become firm,a post forming part of a standard rest plate of a denture is insertedinto the bore until the bottom of the plate comes into contact with theresin.

FIGS. 28 and 29 illustrate a modified insert 350 for use with a restfactor 250 in an equivalent manner to that described above. However,rather than having a multifaceted cut for receiving a wrench in thethreaded bore 352 of insert 350, an external, twelve-sided wrenchelement 354 is secured to the upper end of insert 350. Element 354 hasan outer twelve-sided surface 356 for engaging a wrench with acorresponding internal bore, and a through bore 358 coaxial withthreaded bore 352 but of larger diameter. Element 354 is preferablyformed separately from insert 350 and then pressure bonded or welded toit. A conventional hex wrench may also be used to engage the element354.

Apart from threaded bore 352 and wrench element 354, insert 350 isotherwise identical to that of FIG. 21 and like reference numerals havebeen used as appropriate. It will be understood that a wrench element354 may also be provided on the insert of FIG. 20 or FIG. 22 in place ofthe multi-sided internal cut. This avoids the need for a precisionmachined cut extending through the threads, where the cut must be ofprecise dimensions between those of the inner and outer diameter of thethreads to avoid damaging the threads.

The use of an implant having two or more stabilizing threads on theouter surface of the stem embedded in the jawbone, combined with theinverted cup shape of the lower end of the implant, provides a stableimplant which resists movement even in the case of the very shortversion. Bone growth into the inverted cup and between the threadsprovides a large area of bone to implant contact and osseointegration,providing significant resistance to both lateral and downward forcesboth during and after the osseointegration period.

The non-retentive rest factor can be used to support denturesnon-retentively in the posterior area of the jaw. As a result, thepressure applied to the lower jaw by the denture during mastication isdramatically reduced, reducing the discomfort and bone erosion normallyresulting from denture wear. Although the implant can be installedanywhere in the jaw for retentive or non-retentive engagement with adenture or prosthesis, it is particularly useful for providing supportin posterior regions of the jaw where the jawbone has become eroded. Theaccepted formula for cantilevered dentition attached to anteriorimplants limits the extension of the cantilever over the posterior ridgeto no further than the distance between the plane of the most anteriorimplant and the furthest posterior implant. This results in a lack ofsupport under the molar dentition and an extraction effect on theanterior implants during masticatory function. With the additionalsupport provided by one or more rest factors and implants in posteriorregions of the jaw, the extraction effect is significantly reduced oreliminated. In addition, trauma to the ridge under occlusal load, andthe resulting bone loss, may be reduced.

Although some preferred embodiments of the present invention have beendescribed above by way of example only, it will be understood by thoseskilled in the field that modifications may be made to the disclosedembodiments without departing from the scope of the invention, which isdefined by the appended claims.

I claim:
 1. A dental implant assembly for supporting as a rest factorthe distal end of a denture anchored elsewhere in the jaw, comprising:implant means for embedding in the distal region of the jawboneincluding a first part for projecting into and osseointegrating with thejawbone and a second part for projecting up to just above the tissuelevel; the first part being relatively thin and having a predeterminedheight less than the distance between the top of a patient's jawbone atthe implant site and the underlying region of the mandibular nerve; thesecond part having an upper rest surface for supporting but not beingconnected to overlying portions of a denture anchored elsewhere in thejaw to resist biting pressure on the tissue and bone; and the first partcomprising a thin disc-shaped member having a downwardly projectingannular rim and a central spigot projecting downwardly from thedisc-shaped member.
 2. The assembly as claimed in claim 1, wherein saidfirst part comprises an implant member and the second part comprises arest factor releasably securable to the implant member.
 3. The assemblyas claimed in claim 2, including a plurality of implant members in arange of different sizes, the members having outer diameters in therange from about 4.25 to 6 mm.
 4. The assembly as claimed in claim 2,including a plurality of implant members in a range of different heightsfrom 2 to 4.5 mm.
 5. The assembly as claimed in claim 1 wherein thefirst part has a height no greater than its diameter.
 6. The assembly asclaimed in claim 1, wherein the first part has a selected diameterslightly less than the width of the alveolar crest at the implant site.7. The assembly as claimed in claim 6, wherein the selected diameter is1 mm less than the width of the alveolar crest.
 8. The assembly asclaimed in claim 1, wherein the first part has a height no greater than4.5 mm.
 9. The assembly as claimed in claim 8, wherein the first parthas a height of between 2 to 4.5 mm.
 10. The assembly as claimed inclaim 1, wherein the rest surface has a shaped, cooperating area fornon-retentive seating engagement with a correspondingly shaped matingsurface on an overlying denture.
 11. The assembly as claimed in claim 1,wherein the first part has a central bore in an upper face of thedisc-shaped member.
 12. The assembly as claimed in claim 11, wherein thesecond part comprises a shaft portion for engagement in the central boreof the implant member and a solid head portion projecting upwardly fromthe shaft portion.
 13. The assembly as claimed in claim 12, wherein theextension portion is generally cylindrical with an upper, convex restsurface.
 14. The assembly as claimed in claim 11, wherein the bore hasan upper threaded portion and a non-threaded, lower extension portion,the second part having a shaft portion with a corresponding threadedupper portion and non-threaded lower portion for mating engagement insaid implant member bore.
 15. The assembly as claimed in claim 14,wherein said implant member bore and rest factor shaft portion haveupper ends having matching tapers.
 16. A dental implant assembly forsupporting as a rest factor the distal end of a denture anchoredelsewhere in the jaw, comprising: implant means for embedding in thedistal region of the jawbone including a first part for projecting intoand osseointegrating with the jawbone and a second part for projectingup to just above the tissue level; the first part being relatively thinand having a predetermined height less than the distance between the topof a patient's jawbone at the implant site and the underlying region ofthe mandibular nerve; the second part having an upper rest surface forsupporting but not being connected to overlying portions of a dentureanchored elsewhere in the jaw to resist biting pressure on the tissueand bone; the first part comprising a thin disc-shaped member having adownwardly projecting annular rim; and axially extending spaced groovesin the outer surface of the annular rim for restricting rotation of theembedded implant.
 17. A dental implant assembly for supporting as a restfactor the distal end of a denture anchored elsewhere in the jaw,comprising; implant means for embedding in the distal region of thejawbone including a first part for projecting into and osseointegratingwith the jawbone and a second part for projecting up to just above thetissue level; the first part being relatively thin and having apredetermined height less than the distance between the top of apatient's jawbone at the implant site and the underlying region of themandibular nerve; the second part having an upper rest surface forsupporting but not being connected to overlying portions of a dentureanchored elsewhere in the jaw to resist biting pressure on the tissueand bone; the first part comprising a generally cylindrical memberhaving a downwardly depending annular rim, and a central spigotprojecting downwardly from the cylindrical member, the central spigothaving a length in the range from 1 to 2 mm.
 18. A combined denture andsupport assembly, comprising: a denture of more than one tooth; ananchor securing one end of the denture to the jawbone; support means forfreely supporting a distal region of the denture at a selected locationto the rear of the first bicuspid, the support means comprising a firstpart for embedding in the jawbone at the selected location and a secondpart for projecting up to at least the tissue level, the second parthaving an upper rest surface for supporting but not being connected tothe denture and for accepting biting pressure from the denture; and thefirst part comprising a thin disc-shaped member having a downwardlyprojecting annular rim and a central spigot depending downwardly fromthe disc-shaped member to define an annular cavity between the spigotand rim.
 19. The assembly as claimed in claim 18, wherein said firstpart extends no more than about 4 mm into the jawbone.
 20. The assemblyas claimed in claim 19, wherein said first part has a diameter in therange from 4 mm to 6 mm.
 21. The assembly as claimed in claim 18,wherein said second part extends up to between ½ to 1 mm above thetissue level.
 22. The assembly as claimed in claim 18, wherein the firstpart comprises a flat, disc-shaped member having a downwardly projectingperipheral rim.
 23. The assembly as claimed in claim 22, wherein thefirst part has a height of 2 mm and the rim projects 1 mm below theundersurface of the disc-shaped member.
 24. A dental implant assemblyfor embedding in the jawbone of a patient, comprising: an implant memberfor embedding in and osseointegrating with the jawbone of a patient at aselected site; an attachment member for securing to the implant memberafter osseointegration; the implant member having anexternally-unthreaded upper end portion with and a central bore, theattachment member and central bore of the implant member havinginterengageable securing means for releasably securing the attachmentmember to the implant member, and a an externally-threaded stem portiondepending downwardly from the upper end portion for engagement with arecess in the jawbone of shape and dimensions matching those of the stemportion; the stem portion having an annular indent at its lower end; thestem portion having outwardly projecting screw threads wherein thespacing between adjacent threads is substantially the same, and theheight of said outwardly projecting screw threads, is substantially thesame, over the entire threaded length of said stem portion; and thethreads comprising multiple lead threads.
 25. The assembly as claimed inclaim 24, wherein the threads comprise triple lead threads.
 26. Theassembly as claimed in claim 24, wherein the threads comprise quadruplelead threads.
 27. The assembly as claimed in claim 25, wherein thethreads are of square cross-section.
 28. The assembly as claimed inclaim 24, wherein the spacing between adjacent threads is at least 1 mmand the threads run parallel to one another.
 29. The assembly as claimedin claim 28, wherein the gap between adjacent threads is larger than thewidth of each individual thread.
 30. The assembly as claimed in claim24, wherein the threads project out to a distance of ½ mm from thesurface of the stem portion.
 31. The assembly as claimed in claim 24,wherein the stem portion has an upper end and a lower end and thethreads start at a position spaced below the upper end of the stemportion.
 32. The assembly as claimed in claim 31, wherein the threadsterminate at a position spaced above the lower end of the stem portion.33. The assembly as claimed in claim 24, wherein the central boreprojects downwardly into the stem portion of the implant member.
 34. Theassembly as claimed in claim 24, wherein the upper end portion of theimplant member comprises an annular ring and an inwardly tapering rimprojecting upwardly from the annular ring.
 35. The assembly as claimedin claim 24, including a wrench element projecting from the upper endportion of said insert member for engagement with a tool for insertingthe implant member, the wrench element having a bore aligned with thecentral bore of said insert member.
 36. The assembly as claimed in claim35, wherein the wrench element has a twelve-sided outer wrench engagingsurface.
 37. The assembly as claimed in claim 24, including a pluralityof implant members in a range of different heights from 2 to 13 mm. 38.A dental implant assembly for embedding in the jawbone of patient,comprising: an implant member for embedding in and osseointegrating withthe jawbone of a patient at a selected site; an attachment member forsecuring to the implant member after osseointegration; the implantmember having an upper end portion with a central bore, the attachmentmember and central bore of the implant member having interengageablesecuring means for releasably securing the attachment member to theimplant member, and a stem portion depending downwardly from the upperend portion for engagement with a recess in the jawbone of shape anddimensions matching those of the stem portion; the stem portion havingan annular indent at its lower end; and the attachment member comprisinga rest factor member having an upper rest surface for supporting, butnot being connected to, overlying portions of a denture anchoredelsewhere in the jaw, the rest factor member having an upper,dome-shaped portion and a downwardly depending stem portion forengagement in said implant member bore, the rest surface comprising theupper end of said dome-shaped portion.
 39. The assembly as claimed inclaim 38, including a plurality of rest factor members havingdome-shaped portions of different heights in the range from 1.5 mm. to3.5 mm.
 40. The assembly as claimed in claim 39, in which said restfactor members have total heights in the range from 2 mm. to 4.5 mm. 41.The assembly as claimed in claim 38, wherein said dome-shaped portionhas a hex indent at its upper end for engagement with a hex tool forinsertion of the rest factor member into an implant member bore.
 42. Animplant member for embedding in a patient's jawbone for attachment toother dental devices, the member comprising: a generally cylindricalbody having an externally-unthreaded upper end portion for projectingabove the jawbone,and,a extending downwardly from said upper endportion, an externally-threaded lower stem portion for insertion in arecess prepared in a patient's jawbone; the upper end portionsaidimplant member having a central bore for insertion of a portion of adental device to be attached to the implant member; and the lower endstem portion having an outer cylindrical surface and a plurality ofmultiple lead threads projecting outwardly from the outer surface, saidmultiple lead threads extending over said outer surface and wherein thespacing between adjacent threads is substantially the same, and theheight of said outwardly projecting threads is substantially the same,over the entire threaded surface.
 43. The implant member as claimed inclaim 42, wherein the upper end portion includes a cylindrical ringportion of diameter greater than said cylindrical surface of the lowerend portion, and an inwardly tapered rim portion projecting upwardlyfrom said ring portion.
 44. The implant member as claimed in claim 43,wherein said ring portion has a diameter equal to the diameter of saidthreads.
 45. The implant member as claimed in claim 43, wherein saidupper end portion has a height less than that of the lower end portion.46. An implant member for embedding in a patient's jawbone forattachment to other dental devices, the member comprising: a body havingan externally-unthreaded upper end portion and, extending downwardlyfrom said upper end portion, an externally-threaded lower portion forinsertion in a recess prepared in a patient's jawbone; said implantmember having an internal site for insertion of a portion of a dentaldevice to be attached to the implant member; and said lower portionhaving an outer surface and multiple lead threads projecting outwardlyfrom said outer surface, said multiple lead threads extending oversubstantially the entire length of said externally-threaded lowerportion and wherein the spacing between adjacent threads issubstantially the same, and the height of said outwardly projectingthreads, is approximately the same, over said substantially entirelength.
 47. The implant member as claimed in claim 46 wherein the numberof said lead threads is two, three, or four.
 48. The implant member asclaimed in claim 46 wherein the pitch of each of said lead threads is inthe range of about 9° to about 17° and the spacing between adjacent leadthreads is at least about one millimeter.
 49. The implant member asclaimed in claim 46 wherein the gaps between said lead threads are abouttwice the width of said lead threads.
 50. The implant member as claimedin claim 46 wherein the length of said implant is in the range of abouttwo millimeters to about thirteen millimeters and the outer diameter ofsaid implant member is in the range of about four millimeters to aboutsix millimeters.
 51. The implant member as claimed in claim 46 whereinsaid implant is coated with a material that improves bone adhesion. 52.An implant member for embedding in a patient's jawbone for attachment toother dental devices, the member comprising: a body having anexternally-unthreaded upper end portion and, extending downwardly fromsaid upper end portion, an externally-threaded lower portion forinsertion in a recess prepared in a patient's jawbone; said implantmember having a central bore for insertion of a portion of a dentaldevice to be attached to the implant member; and saidexternally-threaded lower portion having an outer surface, and at leasttwo separate lead threads protecting outwardly from said outer surface,said multiple lead threads extending over substantially the entirelength of said lower portion and wherein the spacing between adjacentthreads is substantially the same, and the height of said outwardlyprojecting threads, is approximately the same, over said substantiallyentire length.
 53. The implant member as claimed in claim 52 wherein thenumber of said lead threads is two, three, or four.
 54. The implantmember as claimed in claim 52 wherein the pitch of each of said separatelead threads is in the range of about 9° to about 17° and the spacingbetween adjacent lead threads is at least about one millimeter.
 55. Theimplant member as claimed in claim 52 wherein the gaps between saidseparate lead threads are about twice the width of said lead.
 56. Theimplant member as claimed in claim 52 wherein the length of said implantis in the range of about two millimeters to about thirteen millimetersand the outer diameter of said implant member is in the range of aboutfour millimeters to about six millimeters.
 57. The implant member asclaimed in claim 52 wherein said implant is coated with a material thatimproves bone adhesion.
 58. An implant member for embedding in apatient's jaw bone for attachment to other dental devices, the membercomprising: a body having an externally-unthreaded upper end portionand, extending downwardly from said upper end portion, anexternally-threaded lower portion for insertion in a recess prepared ina patient's jawbone; said implant member having a central bore forinsertion of a portion of a dental device to be attached to the implantmember; and said externally-threaded lower portion having an outersurface and at least two threads with separate leads projectingoutwardly from said outer surface, said at least two threads formingmultiple lead threads extending over substantially the entire length ofsaid lower portion and wherein the spacing between adjacent threads isapproximately the same, and the height of said outwardly projectingthreads, is approximately the same, over said substantially entirelength.
 59. The implant member as claimed in claim 58 wherein the numberof said lead threads is two, three, or four.
 60. The implant member asclaimed in claim 58 wherein the pitch of each of said separate leadthreads is in the range of about 9° to about 17° and the spacing betweenadjacent lead threads is at least about one millimeter.
 61. The implantmember as claimed in claim 58 wherein the gaps between said separatelead threads are about twice the width of said lead.
 62. The implantmember as claimed in claim 58 wherein the length of said implant is inthe range of about two millimeters to about thirteen millimeters and theouter diameter of said implant member is in the range of about fourmillimeters to about six millimeters.
 63. The implant member as claimedin claim 58 wherein said implant is coated with a material that improvesbone adhesion.